<style type='text/sass'>

    @mixin border-stroke($val) {
        @if $val == light {
            border: 1px solid black;
        }
        @else if $val == medium {
                  border: 3px solid black;
        }
        @else if $val == heavy {
                  border: 6px solid black;
        }
        @else {
            border: none;
        }
    }

    #box {
        width: 150px;
        height: 150px;
        background-color: red;
        @include border-stroke(medium);
    }
</style>

<div id="box"></div>